A Satellite Positioning System uses a series of Satellites which broadcast coded radio signals that enable a properly equipped user to determine a three-dimensional position and time anywhere on or near the surface of the Earth. The NAVSTAR Global Positioning Systems (GPS) system, operated by the U.S. Department of Defense, is the first Satellite Positioning system widely available to civilian users.
GPS receivers often yield inaccurate positioning results due to errors that corrupt the system. Most of the errors are "common errors." Common errors are errors that are experienced by all the GPS receivers in a local area. Common errors include satellite clock deviation, satellite orbit drift, the effects on the satellite clock caused by the Department of Defense's Selective Availability (SA), and changing radio propagation conditions in the Ionosphere and Troposphere.
To improve the accuracy of GPS receivers, differential GPS (DGPS) was created. Differential GPS (DGPS) operates by identifying common errors and then eliminating the influence of the identified common errors to improve the accuracy of the positioning. To implement differential GPS, a first DGPS receiver must be placed at location for which the position coordinates are accurately known and accepted. The DGPS receiver at an accurately known location is called a DGPS "reference station." The DGPS reference station calculates the difference between the known coordinates and the DGPS reference station calculated coordinates to determine the common error introduced to all GPS receivers in a local area. A second DGPS receiver is then placed in a position that needs to be identified. Using the error correction data from the first DGPS receiver (the reference station), the position of the second DGPS receiver can accurately be determined.
The error correction data determined by the reference station is normally broadcast on a particular radio frequency located in a variety of available bands. A standard for broadcasting such DGPS error corrections is known as Radio Technical Committee Maritime (RTCM) SC-104 format. Mobile GPS receivers near the DGPS reference station can receive the broadcast RTCM SC-104 error correction data and apply the error corrections to improve the accuracy of the mobile GPS receivers.
Differential GPS has greatly improved the accuracy of the satellite based GPS system, but there are significant costs associated with this improved service. For example, differential GPS requires more equipment than just a single GPS receiver. Specifically, differential GPS requires a differential GPS reference station and a mobile DGPS receiver. Such differential GPS reference stations are often very expensive. Furthermore, to effectively use a differential GPS reference station an accurate survey must be performed to position the differential GPS reference station at an accurately known location. A radio transmitter must be coupled to the reference station in order to broadcast the error correction information. It would therefore be desirable to have a system of obtaining differential GPS correction information without all the expense and work of purchasing, installing, and maintaining a differential GPS reference station.